At present, energy is unnecessarily wasted in veneer drying, owing to the fact that the conditions of the drying process vary considerably (veneer quality, degree of filling of the drying machine, etc.) But, the adjustable circulating air vent damper is kept in a position such that the drying machine will not release into the plant hall the gases produced in the drying process. The result hereof is a situation wherein the vent damper is in a theoretical optimum position, as regards heat economy, during short moments only, while the damper is opened unnecessarily wide the greater part of the time, thus letting thermal energy escape from the drying process.
As a result of the foregoing, the water content of the circulating air is at present in a range from 80 to 110 kg H.sub.2 O per kg of dry air. In such case between 30 and 40% of the total energy are used to heat replacement air.
If the vent damper is so controlled that it is constantly in its otimum position (whereby the requirements of replacement air are minimized), it is possible to reduce the amount of heat energy that has to be consumed to heat the replacement air, and theoretically it is possible to reduce above-mentioned 30 to 40% of the total energy used. Since the thermal energy consumption of the drying machine is, depending on its capacity, 15 to 28.times.10.sup.3 Gcal per year, a conservative calculation shows that the annual savings can be 15%, or between 2250 and 4200 Gcal per year. (6000 hrs.times.2.5 m.sup.3 /hr to 6000 hrs.times.5 m.sup.3 /hr).times.0.95 Gcal per m.sup.3 of dry veneer.
With the aid of the present invention, the above-mentioned advantages are gained. The procedure of the invention is characterized in that the pressure in the drying space is monitored and the damper is operated to control the vented air quantity in such manner that there will continuously be a certain predetermined vacuum in the drying space.
When the water content of the circulating air is high, thermal energy is transferred from the circulating air to the veneer with considerably greater efficiency than from dry circulating air. The veneer that is being dried will then heat up rapidly, and the evaporation of the water present in the veneer starts quite clearly sooner than in a dry atmosphere, and the drying of the veneer is speeded up.
An advantageous embodiment of the invention is characterized in that with the aid of the pressure in the drying space the humidity of the circulating air is determined, and the damper is controlled so as to maintain the humidity of the circulating air within 250 to 400 g H.sub.2 O per kg. Keeping the humidity of the circulating air within these limits, one obtains the best results in veneer drying.